Electrolyte sheet with protruding features having undercut angles and method of separating such sheet from its carrier

ABSTRACT

According to one embodiment of the present invention an electrolyte sheet includes a body of varied thickness, the electrolyte sheet having a textured surface with multiple protruding features. The protruding features form an undercut angle with respect to the normal of the electrolyte sheet, the undercut angle being more than 0 degrees and less than 15 degrees.

CROSS-REFERENCE TO RELATED APPLICATIONS BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to inorganic electrolyte sheetssuitable for use in fuel cells and, more particularly to texturedelectrolyte sheets suitable for use in solid oxide fuel cells.

2. Technical Background

U.S. Pat. No. 4,135,040 describes the use of textured electrolyte toincrease ion-conductive surface area. The patent specifies that thiselectrolyte is suitable for use in sodium-sulfur, sodium-halogen,lithium anode type cells and solid state cells. These types of galvanic“battery” cells utilize solid state electrolytes which conduct positiveions, such as sodium or lithium, which are very mobile at lowtemperatures. Such electrolytes are typically fairly thick (over 200micrometers), to maintain good mechanical integrity. This patent doesnot disclose solid oxide fuel cells, does not describe the thickness ofthe electrolyte, nor the dimensions of the features on the texturedsurfaces.

The use of electrolyte materials for solid oxide fuel cells has been thesubject of considerable amount of research in recent years. The typicalcomponents of a solid oxide fuel cell comprise a negatively-chargedoxygen-ion conducting electrolyte sandwiched between two electrodes.Electrical current is generated in such cells by oxidation, at theanode, of a fuel material, for example hydrogen, which reacts withoxygen ions conducted through the electrolyte. Oxygen ions are formed byreduction of molecular oxygen at the cathode.

U.S. Pat. No. 5,085,455 discloses thin, smooth inorganic sinteredsheets. The disclosed sintered sheets have strength and flexibility topermit bending without breaking as well as excellent stability over awide range of temperatures. Some of the disclosed compositions, such asyttria stabilized zirconia YSZ (Y₂O₃—ZrO₂) would be useful aselectrolytes for fuel cells. It is known that at sufficient temperatures(e.g., about 725° C. and above), zirconia electrolytes exhibit goodionic conductance and very low electronic conductance. U.S. Pat. No.5,273,837 describes the use of such compositions to form thermal shockresistant solid oxide fuel cells.

U.S. Patent Publication U.S. 2002/0102450 describes solid electrolytefuel cells which include an improved electrode-electrolyte structure.This structure comprises a solid electrolyte sheet incorporating aplurality of positive and negative electrodes, bonded to opposite sidesof a thin flexible inorganic electrolyte sheet. One example illustratesthat the electrodes do not form continuous layers on electrolyte sheets,but instead define multiple discrete regions or bands. These regions areelectronically connected, by means of electrical conductors in contacttherewith that extend through vias in electrolyte sheet. The vias arefilled with electronically conductive materials.

U.S. Patent Publication U.S. 2001/0044043 describes solid electrolytefuel cells utilizing substantially planar, smooth electrolyte sheet witha roughened interface surface layer. This publication discloseselectrolyte sheet thickness below 45 micrometers. The ceramicelectrolyte sheet is flexible at such thicknesses.

U.S. Pat. No. 6,428,920 describes a porous nanocrystaline interfaceroughened layer placed on top of and sintered to the smooth electrolytesheet (substrate). The porous nano-crystaline roughening layer has arandomly structured surface with submicron surface features (grain sizebelow 1 micron and preferably below 0.5 micrometers) and characterizedby the arithmetic average surface roughness of about 0.2 micrometers.

Electrical conductance of the electrolyte is proportional to itsmaterial conductance times its thickness. That is, the electrolyte'sohmic resistance depends on material properties of the electrolyte andis proportional to the thickness of the electrolyte. Thus, in order toreduce ohmic resistance and to improve electrical conductance,electrolyte thickness must be as thin as possible. However, reduction inelectrolyte thickness results in physical weakening of the electrolyte.For example, a ceramic electrolyte sheet having a thickness below 10micrometers often breaks during handling or processing making processyields relatively low. In addition, a defect in an electrolyte sheet maynecessitate a replacement of entire electrolyte structure.

SUMMARY OF THE INVENTION

According to one aspect of the present invention an electrolyte sheetcomprises a body of varied thickness. This electrolyte sheet has atextured surface with multiple protruding features, the protrudingfeatures forming an undercut angle with respect to the normal of saidelectrolyte sheet, the undercut angle being more than 0 degrees and lessthan 15 degrees.

According to one embodiment the a method for separating a greenelectrolyte sheet from its carrier comprises the steps of: (a) placingthe green sheet and its carrier on a vacuum table such that said greensheet is situated adjacent to said vacuum table; (b) applying enoughsuction force to said green sheet to keep said green sheet on saidtable; (c) heating said carrier to a temperature above 30° C.; and (d)lifting said carrier from said green sheet, thus separating said greenelectrolyte sheet from said carrier.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present exemplary embodiments of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as it isclaimed. The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate various embodimentsof the invention, and together with the description serve to explain theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary electrolyte sheet ofone embodiment of the present invention.

FIG. 2 illustrates a cross sectional view of the electrolyte sheet ofFIG. 1.

FIG. 3 illustrates schematically alternative spacings of theindentations of the electrolyte sheet of FIG. 1.

FIGS. 4A and 4B illustrate schematically that the indentations on thesurface of the electrolyte sheet may also be textured.

FIGS. 5A and 5B illustrate an electrode-electrolyte assembly with eightcells connected in series through the interconnect vias.

FIGS. 6A and 6B illustrate schematically two different ways of making atextured green sheet utilizing a textured or patterned surface.

FIG. 7 illustrates schematically a method of making a textured greensheet utilizing a textured or patterned roller.

FIG. 8A illustrates schematically a device for making a textured greensheet utilizing two rollers.

FIG. 8B illustrates schematically a device for corrugating making atextured green sheet utilizing two rollers.

FIGS. 9A-9C are schematic illustrations of exemplary electrolyte sheetswith differently textured surfaces.

FIG. 10A illustrates schematically one mechanism for separating atextured green sheet from its carrier.

FIG. 10B illustrates schematically another mechanism for separating atextured green sheet from its carrier.

FIG. 11A illustrates one example of the electrolyte sheet of the presentinvention.

FIG. 11B illustrates the cross-sectional view of the electrolyte sheetof FIG. 11A.

FIG. 12 illustrates schematically a crossed single cell that utilizes anelectrolyte sheet of FIGS. 11A and 11B.

FIG. 13A illustrates an exemplary electrolyte sheet with thicknessvariations for control of mechanical flexure.

FIG. 13B illustrates a cross-section of the portion of the electrolytesheet of FIG. 13A.

FIG. 14 illustrates schematically another example of electrolyte sheetwith thickness variations.

FIG. 15 illustrates a top view of yet another exemplary electrolytesheet, as seen under a microscope.

FIGS. 16A and 16B illustrate schematically two more examples of atextured electrolyte sheet with a textured surface.

FIG. 16C illustrates a cross-section of a region of the texturedelectrolyte sheets illustrated in FIGS. 16A and 16B.

FIG. 17A is a schematic illustration of an electrolyte sheet mounted ina frame and situated between air and fuel.

FIG. 17B illustrates schematically a cross-section a texturedelectrolyte sheet that has relatively thick central region.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Whenever possible, the same reference numeralswill be used throughout the drawings to refer to the same or like parts.One exemplary embodiment of the inorganic electrolyte sheet of thepresent invention is shown in FIG. 1, and is designated generallythroughout by the reference numeral 10. The electrolyte sheet 10 is athin ceramic sheet, with two opposite major surfaces 20, 22 and anaverage thickness t. At least one of these surfaces, for example surface20, is textured. The surface 20 of the electrolyte sheet 10 of FIG. 1includes multiple features 25 or indentations 30. It is preferable thatthe indentations 30 be periodically arranged on the surface of theelectrolyte sheet. However, the indentations may also be in anon-periodic arrangement.

A cross-section of a portion of the electrolyte sheet 10 is illustratedschematically in FIG. 2. The features 25 of the textured surface 20 mayhave different shapes, as showing, for example in FIG. 3. It ispreferable that the height of the features is at larger than 0.3micrometers. It is even more preferable that the height of the featuresbe in the range of 0.5 to 50 micrometers.

The indentations 30 may be wider than they are deep and may be spacedapart by distances greater than their width W (or greatest dimension).Alternatively, the indentations 30 may be spaced apart by distancesequal to or smaller than their widths. This is shown schematically, forexample, in FIG. 3. The indentations may also be micro-textured as shownin FIGS. 4A and 4B. For example, indentations 30 may have an averagedepth and width of 5 micrometers. These indentations 30 may includegrooves or other structure (for example, 0.5 micrometers deep) on theirbottom surfaces.

The indentations 30 reduce the average thickness t of the electrolytesheet 10, therefore reducing its ohmic resistance and increasing itsionic conductance, without significantly reducing the mechanicalproperties of the electrolyte sheet. This decrease in ohmic resistanceand the increase in ionic conductance advantageously enables theelectrolyte sheet 10 to operate at relatively low temperatures (i.e.,below 725° C.). Thus, one may choose to utilize the electrolyte sheet ofthe present invention in the temperature ranges of 600° C. to 725° C.,as well as in the conventional temperature ranges of 725° C. to 850° C.

It is preferred that the electrolyte sheet be thin enough so that theelectrolyte's ohmic resistance be less than about 0.5 ohm/cm² and morepreferably less than 0.2 ohm/cm². In addition, the indentations orsurface texturing also advantageously increases surface area of theelectrolyte sheet, thus increasing ionic conductance. Thus, in order todecrease ohmic resistance and to increase ionic conductance of theelectrolyte sheet 10 both sides 20, 22 may be textured.

The electrolyte sheet 10 is has a substantially non-porous (i.e.,substantially without closed pores, porosity being less than 5%) bodyand the thickest part the electrolyte sheet 10 is at least 0.5micrometers greater than the thinnest part of the electrolyte sheet. Itis preferable that porosity is less than 3% and more preferable thatporosity is less than 1%. It is also preferable that the difference Δtbetween the thinnest and the thickest part of the electrolyte sheet 10be between 0.5 micrometers and 90% of the average thickness t, or evenmore preferable between 1 micrometer 40 micrometers, and most preferablethat it be between 1 micrometer and 20 micrometers. It is even morepreferable that this thickness difference Δt be between 2 micrometers 15micrometers. It is most preferable that this thickness difference be 3to 10 micrometers. The electrolyte sheet 10 preferably has an averagethickness t that is greater than 4 micrometers and less than 100micrometers, preferably less than 45 micrometers, more preferablybetween 4 micrometers and 30 micrometers, and most preferably between 5micrometers and 18 micrometers. Lower average thickness is alsopossible. The lower limit of thickness is simply the minimum thicknessrequired to render the structure amenable to handling without breakage.It is preferable that the thin areas of the electrolyte sheet be lessthan 20 micrometers thin, preferably less than 15 micrometers thin andmore preferably less than 10 micrometers thin. It is preferable that theelectrolyte sheet body is a monolithic body (i.e., a body produced asone piece instead of multiple layers of different porosity that havebeen sintered together).

The thin, textured electrolyte sheets such as those described above canbe advantageously utilized in the manufacture of solid oxide fuel cells.Thus, according to one embodiment of the present invention a solid oxideelectrode/electrolyte assembly 50 of a fuel cell comprises: (a) a thinceramic electrolyte sheet 10 of varied thickness, with an averagethickness between 3 micrometers and 30 micrometers; (b) at least onecathode 52 disposed on a first surface 20 of the electrolyte sheet 10;and (c) at least one anode 54 disposed opposite the cathode 52, on asecond surface 22 of the electrolyte sheet 10; wherein the electrolytesheet 10 has a thickness variation of at least 2 micrometers. Such anassembly is illustrated in FIGS. 5A and 5B.

More particularly, FIGS. 5A and 5B show two views of self supportingzirconia-3 mole % yttria electrolyte sheet 10 supporting electrodes 52,54 in the form of rectangular segments connected through small vias(holes) 56 in the electrolyte sheet 10. The top plane of theelectrode-electrolyte assembly is shown in FIG. 5A. FIG. 5B is aschematic elevational cross sectional view of a five-cell section of theelectrode-electrolyte assembly shown in FIG. 5A. According to thisembodiment the electrode/electrolyte assembly 50 includes a plurality ofanode-cathode pairs 52, 54. The anode-cathode pairs 52, 54 are separatedfrom one another by via galleries 55. The via galleries 55 include aplurality of interconnects (called “via interconnects”) 56′ situated inthe vias 56. These interconnects 56′ conduct electronic current from theanode of one cell to the cathode of an adjacent cell. It is preferablethat the body of the electrolyte sheet 10 which is located under theelectrodes (anode(s) and cathode(s)) is relatively thin. That is, it ispreferable that of 50% and more preferably 75% of the area under theelectrodes be thinned. This design is notable for the absence ofexpensive interconnect plates.

Thin electrolyte sheets can be formed in the green state in a moldedconfiguration and subsequently sintered to form an electrolyte sheetwith a large measure of flexibility. Preparation of green (unfired)material is known in the art and is described, for example in U.S. Pat.No. 4,710,227. More specifically, this patent discloses the preparationof thin flexible “green” (unfired) tapes from solutions, the tapes beingcoated and cut, stacked and fired to form thin-dielectric capacitors.This type of process is further described in published Europeanapplications EP 0302972 and EP 0317676. Thus, in order to manufacture athin, textured, electrolyte of the present invention a thin texturedsheet or layer comprising the green pre-sinterd material, is firstproduced. The green pre-sintered material is then sintered to provide atextured, sintered ceramic sheet with a flexibility sufficient to permita high degree of bending without breakage under an applied force.Flexibility in the sintered ceramic sheets is sufficient to permitbending to an effective radius of curvature of less than 20 centimetersor some equivalent measure, preferably less than 5 centimeters or someequivalent measure, more preferably less than 1 centimeter or someequivalent measure.

By an “effective” radius of curvature is meant that radius of curvaturewhich may be locally generated by bending in a sintered body in additionto any natural or inherent curvature provided in the sinteredconfiguration of the material. Thus, the resultant curved sinteredceramic electrolyte sheets can be further bent, straightened, or bent toreverse curvature without breakage.

The flexibility of the electrolyte sheet will depend, to a largemeasure, on layer thickness and, therefore, can be tailored as such fora specific use. Generally, the thicker the electrolyte sheet the lessflexible it becomes. Thin electrolyte sheets are flexible to the pointwhere toughened and hardened sintered ceramic electrolyte sheet may bendwithout breaking to the bent radius of less than 10 mm. Such flexibilityis advantageous when the electrolyte sheet is used in conjunction withelectrodes and/or frames that have dis-similar coefficients of thermalexpansion and/or thermal masses.

The texturing of one or both surfaces 20, 22 can be accomplished invarious manners prior to sintering of the electrolyte sheet. Forexample, textured electrolyte sheets can be produced by providing agreen (i.e., un-sintered) sheet of solid, negative ion-conducting (e.g.,O₂ ⁻) material (for example, zirconia-3 mole % yttria, referred toherein as 3YSZ), texturing at least one face of this green sheet andthen sintering the textured green sheet to provide a solid ionconductive electrolyte sheet with thickness variations from 0.5micrometers to 40 micrometers. It is preferable that the sintering isdone at temperatures above 1000° C., more preferably above 1300° C., forabout 1-3 hours. For example, a method of making a textured electrolytesheet includes the steps of: (a) providing a green sheet; (b) texturingthe green sheet such that it has varied thickness to provide at least0.5 micrometer variations in its thickness; (c) sintering the textured,green sheet to provide an electrolyte sheet with a substantiallynon-porous body, the non porous body having a textured surface withmultiple indentations therein, wherein the thickest part of theelectrolyte sheet is at least 0.5 micrometers greater than the thinnestpart of the electrolyte sheet. It is preferable that the green sheet andthe resulting electrolyte sheet has a thickness variation of at least 2micrometers.

There are several methods of producing the desired surface texturing ina flexible ceramic. One method involves tape casting or web coating overa patterned substrate carrier 100. The substrate carrier 100 can bepatterned, for example, by two methods. One, is to have a movingsubstrate carrier (for example, Mylar®, a continuous belt of steel,Teflon®, a Teflon® coated fabric, polyethylene) embossed with thereverse pattern 100A of the desired indentations, before tape casting orweb coating the green material 102 (also referred to as slip) on thesubstrate carrier. This is illustrated schematically in FIG. 6A. Forexample, applicants were able to successfully emboss 1.5 μm deepfeatures onto a 125 μm Teflon® substrate carrier by applying 100 psi at160° C. with a static press for dwell time of about 1 minute. The slipwas then cast on the embossed carrier using a doctor blade, forming agreen sheet. When dried, the resultant green sheet 103 successfullyretains the desired pattern. Smaller features on the electrolyte surfacehave been also produced by this method.

Another texturing method is to use a second layer 104 (for example, apolymer layer) on a flat substrate carrier (also referred to as basesubstrate herein) and patterning the second layer 104 instead of theflat base substrate 100. This is shown schematically in FIG. 6B. If thelayer 104 is made of polymer, this polymer may be, for example,polymethyl methacrylate in a solution of ethyl acetate. When the solventdries the polymer becomes a pliable solid material which is easy topattern. The polymer layer 104 is then patterned, either by embossing(for example, by embossed roller or platen) or by stripping selectedareas of the polymer from the substrate carrier 100 (i.e., the basesubstrate), leaving a pattern of the fugitive polymer. The high spots inthe polymer pattern would correspond to the thin areas of theelectrolyte sheet 10. If the second layer, i.e., layer 104 is embossed,it is preferable that embossing is done at temperatures between 30° C.and 170° C. This is because the Tg temperature (i.e., the softeningtemperature of the polymer sheet, or the second layer 104 (immediatecarrier)) is likely to be in this range, and softening of the secondlayer 104 can make the embossing of this second layer 104 easier. Forexample, if the second layer 104 is PMMA (poly methyl methacrylate),this material's Tg will be about 110° C. If the second layer 104 is polybutyl methacrylate, this material's Tg will be about 30° C.

The texturing of the electrolyte sheet may be achieved, for example, bymolding or embossing the green sheet, when the green sheet is placed ina suitable mold or die (preferably with periodic depressions) to formdesired surface indentations. Alternatively, sufficiently thin ceramicsheets can be also reformed through a process of superplasticdeformation at high temperatures below their melting points. However,more effective and economic electrolyte sheet patterning can be achievedthrough the process of reshaping unfired green sheet at above roomtemperature (20° C.) and below 200° C., prior to sintering. For,example, the green sheet may also be patterned at a temperatures of 25°C., 30° C., 40° C., 50° C., 60° C., 70° C., 80° C., 90° C., 100° C.,110° C., 125° C., 150° C. or 200° C. The preferred temperature range is50° C. to 100° C. More specifically, it is preferred that the greensheet be patterned at a temperature that is below Tg temperature of itsimmediate carrier (i.e., the layer directly under the green sheet). Morespecifically, embossing would ideally occur at a temperature where thegreen sheet's elastic modulus is lower than the immediate carrier's 100or 104 elastic modulus. In this case, the softer green sheet will bedeformed more than the immediate carrier (100 or 104). The embossingpressure is preferably 150 psi or less, preferably 70-100 psi. Otherpressures may also be utilized.

Another approach is to web coat or tape cast onto a uniform substratecarrier 100 that is drawn over a textured roller 105. This is shownschematically in FIG. 7. The coating die 106 dispenses the green slip102 which forms a film when the slip 102 comes into contact with themoving substrate carrier 100. When web coating is utilized, the coatingdie 106 dispensing the green slip 102 will be typically far enough(greater than 25 micrometers and preferably greater than 50 micrometers)off the substrate carrier 100 that the green coating may not fullyreflect the underlying texture of the roller. A tape casting “doctorblade” 107 may be (optionally) placed 1 to 30 cm downstream from the webcoating die 106, and with proper clearance (less than 50 micrometers andpreferably 10 to 25 micrometers), it can remove the green material 102from the higher areas of the green sheet, thus producing the desiredthinner areas in the now textured green sheet 103. The green slipcontains a relatively large amount of volatile liquid material (20-65 wt% and preferably 30-40 wt %) that boil below 250° C. The volatile liquidmaterial may include at least one solvent and/or at least oneplasticizer. An example of such material is butanol, ethanol, ethylacetate. Other materials may also be utilized. The volatile material isat least partially evaporated from the green sheet either by air flow orheat. It is preferable to emboss the green sheet while it contains 10%to 50% of the initial volatile material. For example, if initially thegreen slip contained 30 wt % of volatile liquid material, after partialevaporation the green sheet may contain only 10% of this material, or 3wt %. These volatile liquid materials will be described in more detailfurther in the specification.

An alternative method is to run a cast, but unfired, green sheet throughembossing rollers 105′, 105″. A typical roller diameter may be 10 cm to20 cm. At least one of these rollers 105′, 105″ is patterned. This isillustrated schematically in FIG. 8. More specifically, the green slip102 is cast onto the carrier substrate 100. The carrier substrate 100may have a smooth surface, which would initially result in a smoothgreen sheet. A predetermined amount of the volatile material (initiallypresent in the green slip) will be evaporated, preferably leaving someresidual volatile material in the green sheet. The high areas of therollers 105′, 105″ will squeeze the green sheet, thinning some areas.The applied pressure is 5 psi to 110 psi and preferably 70 psi to 100psi. The rollers 105′, 105″ are maintained at a temperature of 20° C. to125° C., preferably 30° C. to 100° C. and more preferably 50° C. to 100°C. Temperatures of below 100° C. are preferred in order to utilize lowcost water heating. Higher temperatures using other heating elements canalso be utilized. The higher temperatures improve embossing capabilitybecause heated green sheet softens or has a lower viscosity duringembossing. The textured green sheet 103 is then wound upon the take-uproller 108. When working with green (i.e., unfired) sheets of 15 to 30micrometers thick, it is preferable to have the green sheet run throughthe two rollers 105′, 105″ while it is supported on a substrate carrierthat is as thick or thicker than the green sheet. It is noted that inorder to emboss both sides of the green sheet, two patterned rollers maybe utilized. These rollers may have different patterns and the patternsmay be either aligned or not aligned with one another. The green sheetmay be also placed between two polymer carrier sheets during embossing.However, in this case the thickness of the carrier is relatively thin,ie. about the same or thinner than that of the green sheetAlternatively, embossing of the green sheet may be done by removing thegreen sheet from the carrier and running the green sheet, without thecarrier, through the embossing mechanism (for example, rollers 105′,105″).

If a corrugated electrolyte sheet is desired, the corrugation pattern(non-planar features that are grater than the thickness of thegreen/electrolyte sheet) may also be achieved by moving the green sheetand its carrier between two patterned rollers (preferably heated 30°C.-100° C., and more preferably 50° C. to 100° C.), instead of using onesmooth roller and one patterned roller. To produce corrugation, the twopatterned rollers should have matching patterns such that theprotrusions on one roller will correspond to indentations on the otherroller and the green material will be bent according to the patterns. Agreen sheet that has been textured may be sent through the corrugationrollers to produce a textured corrugated green sheet. Furthermore, asmentioned above, the green sheet may be fed through the rollers 105′,105″, without feeding the carrier sheet through the rollers as well.

It is preferable that green sheet will not be bound too strongly to itscarrier, otherwise, when the drying is complete, the green sheet will bedifficult to separate from its carrier. However, certain degree ofadhesion is desired, or the patterned roller 105′ may cause delaminatingwhile embossing. An exemplary material suitable for the rollers isTeflon®. Other materials may also be utilized. Rollers coated with arelease agent such as methyl cellulose, oil, or wax may also beutilized. Different types of wax and oils may be utilized, for example,parapheric, long chain alcohols (C-14 or longer), non-crystaline,crystalline, micro-crystaline and saturated or unsaturated acids such asoleic or steric acids.

Also, the carrier and/or roller(s) should be rigid enough to allowembossing of the green sheet under pressure. If it is too soft, thepatterned tool may emboss features into the carrier instead of the greensheet. Thus, if the embossing of the carrier along with the green sheetis not desired, it is preferred that Tg of the green sheet be lower thanthe Tg of the carrier. It is preferred that the elastic modulus orviscosity of the green sheet be lower than that of the carrier.

Finally, in order to separate (release) the textured green sheet 103from its carrier 100 and/or 104, one may utilize one of the twofollowing approaches. The carrier may be heated, for example to 50° C.or higher, preferably to 75° C. or above, and more preferably to about100° C. while utilizing a vacuum table method disclosed, for example, inU.S. patent Application No. 20020174935. The heated Teflon carrier tendsto delaminate from the textured green sheet 103 (due to mismatch of theCoefficients of Thermal Expansion, CTE and/or internal stresses of thecarrier), releasing the textured green sheet 103 from the carrier. Morespecifically, a method for separating a green sheet 103 from its carriermay include the steps of: (a) placing the green sheet and its carrier ona vacuum table such that the green sheet is situated adjacent to saidvacuum table; (b) applying enough suction force to the green sheet tokeep the green sheet on the table; (c) heating the carrier to atemperature above 30° C. (and preferably to 50° C. to 150° C.); and (d)lifting the carrier from the green sheet, thus, separating the greenelectrolyte sheet from the carrier. This is shown, schematically on FIG.10A. This approach is especially useful when the green sheet 103 has atextured surface such that the projecting features 25 on the texturedsurface of the green sheet are wider on top than at the base or do nothave the 90° walls, or have undercut features, because the regularvacuum assisted release method does not work as well with green sheethaving such features.

An electrolyte sheet with such features is illustrated schematically inFIGS. 9A-C. As seen in FIGS. 9A-9C, the protruding features 25 formundercut angles θ with respect to the normal N of the green sheet. Thus,if one surface of the green electrolyte sheet 103 includes multipleprotruding features 25 with undercut angles which are coupled tocomplimentary features in the carrier 100 or 104, the step of liftingthe green sheet away from its carrier unzips the protruding features 25from the complimentary features of the carrier.

The alternative approach is to release the textured green sheet from thecarrier by using two rollers 108, 109 is shown schematically in FIG.10B. Roller 108 is the take-up roller for the textured green sheet andis utilized to pull and roll the textured green sheet 103. Roller 109 isa carrier roller and is utilized to pull and roll the carrier 100 and/or104. This approach is especially useful when the textured green sheethas surface features 25 with undercut angles. The roller 108 simplypulls the undercut features 25 out of the interlocking carrier and“un-zips” the green sheet from its carrier.

More specifically, FIG. 10B illustrates a device for separating a greensheet 103 from its carrier which includes: (a) a first roller, the firstroller pulling the textures green sheet 103 from the carrier; and (b) asecond roller displaced by a predetermined distance from the firstroller, the second roller being a take take-up roller for the carrier.In this embodiment, the first roller is a take-up roller for the greensheet 103. The green sheet 103 has a textured surface facing thecarrier. The textured surface of the green sheet 103 has projectingfeatures 25 that interlocks with complementary features of the carrier.The first and said second rollers, together, provide enough force toseparate said projecting features of the green sheet 103 from thecomplementary features of the carrier. The device of FIG. 10 alsoincludes at list one optional strip. This strip is located proximate tothe side of the carrier, and, in conjunction with the rollers providesappropriate angular separation (20 to 90 degrees) between the greensheet 103 and its carrier. Alternatively, an additional roller may beutilized to provide appropriate angular separation between the greensheet 103 and its carrier. Therefore, a method for separating a greenelectrolyte sheet from its carrier may comprise the steps of:

-   -   (a) securing a first portion of the green electrolyte sheet to a        first take up roller;    -   (b) securing a second portion of the carrier to a second take up        roller;    -   (c) transferring the green electrolyte sheet and the carrier        toward the first and the second take up rollers;    -   (d) turning the first roller at to provide enough tension to the        green sheet to separate the green sheet from its carrier; and    -   (e) turning the second roller to at least partially roll up the        carrier.

Other methods may also be utilized to separate the green sheet from itscarrier. It is preferable that the undercut angle θ be less than 15°. Itis more preferable preferably that the under cut angle be 10° or less.Alternatively the angle θ may be larger if the carrier is simply burntduring the sintering step.

It is noted having an electrolyte sheet with a textured surface havingfeatures shown in FIGS. 9A-C is advantageous because when electrodes areapplied to the electrolyte sheet, the electrode material will adhere tothe electrolyte sheet and the above mentioned features of theelectrolyte sheet will make it less likely that the electrodes willseparate or delaminate from the electrolyte sheet during the operationof the fuel cell device (heat cycling).

Therefore, according to one embodiment of the present invention a methodof making an electrolyte sheet comprises the steps of: (a) providing agreen slip on a carrier, the slip containing a relatively liquidvolatile material; (b) spreading the slip upon the carrier to form agreen sheet; (c) at least partially evaporating the liquid volatilematerial; (d) embossing the green sheet with at least 0.5 micrometervariations in its thickness thereby providing textured, green sheet; (d)sintering the textured green sheet to provide an electrolyte sheet withsubstantially non-porous body, the non porous body having a texturedsurface with multiple indentations therein, wherein the thickest part ofthe electrolyte sheet is at least 0.5 micrometers greater than thethinnest part of the electrolyte sheet. For example, the green sheet maybe squeezed between two rollers, wherein at least one of the rollers isa patterned roller. It is preferable to emboss the green sheet while itcontains 10 to 50% of the liquid volatile material.

According to another embodiment a method of making an electrolyte sheetcomprises the steps of: (a) providing a green slip on a patterned orembossed carrier with at least 0.5 micrometer surface variations, saidslip containing a relatively volatile material; (b) spreading said slipon said patterned carrier to form a textured green sheet having at leastone textured surface; (c) at least partially evaporating this material;(d) sintering the textured green sheet to provide an electrolyte sheetwith substantially non-porous body, the non porous body having atextured surface with multiple indentations therein, wherein thethickest part of the electrolyte sheet is at least 0.5 micrometersgreater than the thinnest part of the electrolyte sheet.

It is preferable that at least one of the rollers is at a temperature ofat least 30° C., preferably in the 50° C.-150° C. range and mostpreferably in the 70° C.-100° C. range.

The preferred electrolyte sheets 10 are formed of a polycrystalineceramic selected from a group comprising of partially stabilizedzirconia or stabilized zirconia, the partially stabilized, or stabilizedzirconia, being doped with a dopant selected from the group comprisingof the oxides of Y, Ce, Ca, Mg, Sc, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,Yb, Lu, In, Ti, Sn, Nb, Ta, Mo, W and mixtures thereof. The crystallinegeometries of zirconia such as tetragonal, monoclinic, and/or cubic andtheir combinations are all important physical parameters of thisstructural material. It is most preferable that partially stabilizedzirconia is utilized because of its transformational tougheningproperty. The tetragonal phase transforms into monolithic phase understress and effectively prevents crack formation and propagation. Thus,the electrolyte sheet is more durable, mechanically strong and easier tohandle. The more preferable stabilizing dopants are Y, Yb, Ca, Mg, orSc. Of these dopants Sc, Yb and Y have excellent ionic conductivity.Ceramic sheets can be made tougher by selecting certain tougheningagents known to those skilled in this art. Particularly useful andpreferred toughening agents are the oxides of tantalum and niobium whichcan be advantageously added to the above stabilizers.

Volatile Liquid Materials

The volatile liquid material may include a single solvent or a mixtureof solvents. Each solvent is characterized by a boiling point andevaporation rate. The evaporation rates are disclosed, for example inthe book entitled Organic Solvents, Physical Properties and Methods ofPurification, Fourth Edition, John A Riddick, William B. Bunger andTheodore K. Sakano. John Wiley & Sons 1986. The properties of the pure(i.e., unmixed) solvents serves as a guide for their behavior but theactual evaporation rate of mixtures is be governed by the thermodynamicsof the mixed systems. Examples of three types of suitable volatileliquid materials are described below.

The liquid volatile material for the green slip may also contain atleast one solvent and at least one binder which dissolves in or isdispersed in the solvent. The binder may include, for example, one ormore of the following: poly vinyl alcohol, acrylics, poly vinyl butyral,poly ethylene oxide and polyethylene glycols of various molecularweights, polyvinyl pyrrolidone, cellulosics such ashydroxymethylcellulose, hydroxyethylcellulose andhydroxyproplycellulose, gums such as agar gum and gum Arabic, acrylics,vinylacrylics, acrylic acids, polyacrylamides, starches or othercombinations and permutations of binders known in the art. Alternatelythe green slip may contain an emulsion such as an acrylic emulsion withan aqueous liquid (i.e. a liquid that includes water). Alternately, theliquid volatile material of the green slip may contain one or morebinder precursors which act to form the binder upon heating, drying orexposure to radiation. Such binder precursors are, for example, acrylicssuch as poly methyl methacrylate, or condensation polymers, such aspolyfunctional acids and glycols. In addition, the liquid volatilematerial of the green slip may contain a low vapor pressure, highboiling temperature (above 250° C.), plasticizer such as dibutylphthalate, butyl phenyl phthalate, low molecular weight poly ethyleneglycol, or other plasticizers, which act to lower the glass transitiontemperature (Tg) or improve the plasticity of the particular binder soas to make the green sheet less brittle during handling or subsequentforming. With the use of sufficient plasticizer, the Tg, of the bindersystem (one or more binder and one or more plasticizer) and theeffective Tg of the green sheet may be lowered below the temperature ofembossing so as to enable embossing of the green sheet without fracture.In general, increasing the plasticizer content lowers the Tg of thegreen sheet. However, if the Tg is too low, then the green sheet will bedifficult to handle due to low strength or insufficient resistance todeformation from handling. It is common practice in the dielectricindustry to use a high ratio of plasticizer to binder so as to enablesubsequent lamination and deformation. In the case of green sheet tooreadily deformable to handle at room temperature, the green sheettypically may remain supported by the carrier film throughout theembossing process.

It is preferred that the green sheet be stiff enough to be selfsupporting without deformation for handling after release from thecarrier. If release takes place before embossing it is preferred thatthe green sheet be stiff enough to perform embossing without thecarrier. This may be achieved by maintaining a transition temperaturefor the binder of between −50° C. and 100° C., more preferably between−10° C. and 50° C. (The transition temperature is a temperature at whichthe is a change in the slope of elastic modulus of the material vs.temperature.) This is achieved, for example, in the polyvinylbutyral/dibutyl phthalate binder system by a relatively high ratio ofbinder to plasticizer, typically greater than 0.5 by weight, morepreferably between 1 and 3.5, and more preferably between 1.25 and 2.5.Other ratios may be appropriate for other binder systems. For example,poly butyl methacrylate has a Tg low enough to require little or noplasticizer to obtain a Tg below 30° C. In the polyvinyl butyral/dibutylphthalate system, a a binder to plasisizer ratio of around 2 enablesgood handling of the green sheet but not embossing below 30° C.

DMS measurement of green sheets made using polyvinylbutyral and dibutylphthalate binders show a transition temperature between −10° C. and 30°C. The liquid volatile materials also included the following sovents:methanol and 2-methxy ethanol. Examples are shown in the Table 1, below.These measurement were performed on the green sheet supported by a polymethyl methacrylate fugitive carrier. The transition temperature wasseen as a rise in tan δ and is typically very broad, leading to someuncertainty and a range of values in some cases. For a ratio of 1.25 thetransition temperature was apparently so broad as to not be observablein this measurement. The fugitive carrier transition was also observed.

TABLE 1 First Binder Plasticizer Ratio Ratio transition Sample (g) (g)(binder/plasticizer) Zirconia (g) (binder/zirconia) (° C.) a 3.50 2.801.25 70.15 0.05 Not observed b 5.05 3.37 1.50 84.17 0.06  6 c 4.98 2.452.03 82.98 0.06 15-17

The transition temperatures are thought to be due to the inherent bindersystem softening and the softening of the composite binder/zirconia ofthe green sheet. For this reason, it is advantageous to heat the greensheet to at least 30° C., and more preferably between 50° C. and 100° C.to enable better embossing. The overall stiffness of the green sheet maybe adjusted by adjustment of either the binder/plasticizer ratio or byadjustment of the ration of binder plus plasticizer to ceramic powder(for example, zirconia). In general, raising the ratio ofbinder-plasticizer to ceramic powder will cause the overall green sheetplasticity to more closely resemble that of the binder-plasticizeralone. A binder/ceramic power ration of between 0.01 and 0.2 ispreferred, and more preferred is a ratio of between 0.03 and 0.1. Ratiosof 0.05 and 0.06 are shown in Table 1. The binder/power ratio values arechosen to give sufficient strength and plasticity to the green sheetwhile minimizing its shrinkage on firing.

A lower embossing temperature may also be achieved by the addition ofnon-volatilized residual liquid of intermediate boiling point. In thelatter case it is preferred that a residual amount of solvent remainsafter the initial drying so that it acts to further plasticize thepolymer. This liquid need not be a good solvent for the binder but mustserve to sufficiently plasticize it during the embossing step. Theamount to which the Tg is lowered is determined by both thebinder/plasticizer ratio and by the binder/residual-liquid ratio.Examples of suitable solvents include alcohols, ketones, acetates,ethers, glycols, glycol ethers or solvents with mixed functionality withhigh boiling points and/or low evaporation rates. This can include1-butanol, 2-methyl-1-propanol, 2-pentanol, terpineol or other highboiling alcohols. Preferred are the glycols and glycol ethers such asethylene glycol, propylene glycol, 2-methoxy ethanol, 2-ethoxy ethanol,and other high boiling alcohols. These examples are not meant to limitthe possible combinations of polymer, solvent and plasticizer but toillustrate what may be further known to those versed in the art.

Furthermore, a volatile liquid may comprise a low boiling and/or highevaporation rate solvent. For volatile liquids including polyvinylbutyral as a binder, these solvents are, for example, one or more of thefollowing compounds: methanol, ethanol, 1-propanol, 2-propanol, acetone,methyl ethyl ketone, and other solvents having vapor pressures at 25° C.of five or more torr and boiling points below 100° C. Such solvents areadvantageous because they speed the drying time of the green sheet, andthus increasing troughput.

In summary, the green slip may contain a binder and optionally aplasticizer or high-boiling solvent, and a low-boiling solvent. Byappropriate selection of these materials, the green sheet is hassufficient plasticity to emboss and adequate stiffness for furtherhandling. In one embodiment, a green slip is made using a lowtemperature boiling solvent (i.e., a solvent with a boiling point below100° C.), a high temperature boiling solvent (i.e., a solvent with aboiling point above 100° C.), and a plasticizer. After drying the greensheet at below 100° C. the low temperature boiling solvent has beenremoved, but at least a portion of the high temperature boiling solventremains along with the plasticizer. The green sheet is then be embossedat a temperature below the deformation temperature Tg of the carrier.After embossing, the green sheet is further dried to remove theremaining high temperature boiling solvent and is released from thecarrier by pulling across a sharp support. Upon release the green sheetmay be handled without either brittle failure or unwanted deformation.The green sheet is then placed on an alumina setter and fired orsintered to form the final electrolyte sheet.

The preparation of a green 3YSZ based sheet involves some form ofcasting an appropriate ceramic slip. One such slip is described in thefollowing example.

Preparation of a Ceramic Slip

A ceramic slip is made containing 100 grams of zirconia powder using thecomponents shown in Table 2, below.

TABLE 2 Components of Slip Batch Component Manufacturer Form FunctionBatch mass Zirconia TZ-3Y Tosoh powder ceramic 100.0 g EthanolMallinkrodt liquid solvent 36.36 g 1-Butanol Fisher liquid solvent  8.79g Propylene Glycol Fisher liquid solvent  2.00 g Water (distilled)liquid solvent  2.50 g Emphos PS-21A Witco liquid dispersant  1.00 g

All ingredient bottles are kept tightly closed until used to reducewater pickup. A 250 ml Nalgene® polyethylene plastic container iscleaned by rinsing twice with about 10-20 ml of ethanol or methanol. Thecontainer is then placed in a 70° C. drying oven to evaporate thealcohol. After drying, the container and lid are weighed. The zirconiapowder is weighed out in a weighing boat and set aside. The ethanol ispipetted into the dried Nalgene container. The 1-butanol is thenpipetted into the plastic container. Subsequently, propylene glycol ispipetted into the container. Finally, the water and, then, the EmphosPS-21A are pipetted into the container. About 450 grams of 10 mm TosohTZP-3Y milling media is weighed out and the weight recorded. The mediais then added to the container and the container is gently shaken. Thezirconia powder is then added from the weighing boat. The lid isreplaced onto the container and tightened. The closed container isre-weighed and the total weight of solvents, powder, and dispersant iscalculated. The slip is then subjected to vibratory milling for 72hours, after which the viscosity of the slip is measured.

Two settling steps are performed in order to remove the coarse grainsand narrow the grain size distribution in the slip. A double settlingtechnique provided a good grain size distribution with acceptablematerial losses.

A second 125 ml Nalgene® plastic container and lid are washed and driedas above. The second container and lid is weighed and the weightrecorded. The slip from the milling container is poured into the secondcontainer, leaving the milling media in the original container. The slipand second container with lid is then weighed. The coarse grains areallowed to settle out of the slip for 72 hours. A third container andlid are washed, dried, weighed, and the weight recorded. Carefully, theunsettled slip is pipetted into the third container, being careful notto pick up any of the settled sediment. The pipetted slip with the thirdcontainer with lid are weighed. This slip is then allowed to settle foran additional 24 hours. The residue/sediment in the second container isdried in a ventilated oven at about 90° C. for at least three hours andthe dried residue and container plus lid are weighed.

A fourth 125 ml plastic container and lid are washed and dried as above.The weight of this fourth lid and container is then recorded. Again, theslip from the third (24 hour settling) container is pipetted into thefourth container, being careful not to pick up any of the settledresidue with the pipette. The fourth container and slip are weighed andthe weight recorded. The reside is dried in the third container, asabove, and then weighed. From the recorded weights, it can be determinedhow much ceramic powder is left in the fourth container.

A weak flocculating agent, glacial acetic acid, a plasticizer, and abinder are added to the ceramic powder present in the remaining slip.The components used for flocculation and binding, reported in weightpercent in excess of the weight of the remaining ceramic powder, areshown in Table 3, as follows:

TABLE 3 Components Used For Flocculation and For Binder System ComponentManufacturer Form Function Batch mass Glacial Acetic Malinckrodt liquidflocculant   1 wt % of Acid remaining ceramic powder Isopropyl alcoholFisher liquid acid dilution   1 wt % Dibutyl-Pthalate Aldrich liquidplasticizer 3.5 wt % Polyvinyl Butyral Monsanto powder binder   6 wt %

A 50/50 wt % solution of glacial acetic acid in isopropyl alcohol ismade. 2 wt % (in excess of the weight of the remaining ceramic powder)of this solution is pipetted into the slip in the fourth container. Thelid is replaced and the container is gently shaken. Next, 3.5 wt % (inexcess of the weight of the remaining ceramic powder) ofdibutyl-pthalate is pipetted into the slip in the fourth container. Thelid is replaced and the container is gently shaken. Using a weighingboat, 6 wt % (of the remaining ceramic powder) of polyvinyl butyral isweighed out and poured into the slip. The lid is replaced and thecontainer is gently shaken. This container is then placed on a paintshaker type of device for at least 15 minutes to fully dissolve thebinder. Two clean zirconia milling media are put into the container andthe container is placed on a roller mill at low speed for three days.

The use of a polymer (polymethyl methacrylate) base layer in the tapecasting procedures is advantageous because it makes the thin greenmaterial easier to handle. To provide such a layer, a fugitive polymersolution was prepared in a polyethylene bottle by dissolving 40 parts byweight polymethyl methacrylate in 60 parts of ethyl acetate. Thesolution was placed on a roller mill to mix. The acrylic polymersolution thus provided was then cast onto a substrate carrier using adoctor blade to form thin acrylic sheet. The polymer-coated substratecarrier was then placed in a 60° to 70° C. drying oven for 30 to 60minutes.

It is noted that at sufficient temperatures (e.g., about 600° C. andabove), zirconia based thin electrolyte sheets exhibit good ionicconductance (smaller than 0.001 S/cm) and very low electronicconductance (less than 1×10⁻⁶ S/cm) It is noted that electrolyte ionicconductivity increases with higher operating temperatures, but thechoice of stable materials used (for example, metals useful formanifolding) becomes increasingly limited since inexpensive metal alloyswill oxidize above about 850° C. Therefore, it is preferable that fuelcells which include electrolyte sheets of the present invention operatebetween 600° C. and 850° C.

EXAMPLES

The invention will be further clarified by the following examples.

Example 1

FIG. 11A illustrates one example of the electrolyte sheet of the presentinvention. FIG. 11B illustrates the cross-sectional view of theelectrolyte sheet of FIG. 11A. The electrolyte sheet 10 of this examplehas two textured surfaces 20, 22. This electrolyte sheet was made asfollows:

-   -   i. In a filtered air “clean” environment, Teflon® coated cloth        (150 micron-Ultra Premium Grade PTFE coated fiberglass fabric,        available from CS Hyde Co., Lake Villa Ill., USA) is smoothed        out on a glass plate and then attached to the glass plate with        tape. The Teflon® coated cloth has an existing micro texture        (weave).    -   ii. A slip of zirconia—3 mole % yttria powder is coated on the        Teflon® coated cloth using a tape casting “doctor” blade with a        50 micron gap and a 15 cm width, forming a green sheet.    -   iii. The micro-textured green sheet (i.e. the green sheet with        the size of features 25 between 0.5 and 50 microns) was dried        for ½ hour at room temperature under a plastic cover that        encased the drying green sheet, with about a 2 mm gap along the        width of the ceramic sheet but no gap along the long edges.    -   iv. The micro-textured green sheet was then dried in an oven at        60° C. for 1 hour.    -   v. Finally, the micro-textured green ceramic sheet was sintered        at 1430° C. for 2 hours.

After sintering the micro-texturing (periodic variations in thickness)of the electrolyte sheet 10 was observed via SEM (Scanning ElectronMicroscope). The SEM observation showed that the maximum thickness ofthe micro textured sheet was about 23 micrometers and the thickness ofthe thin areas was about 17 micrometers. The sintered electrolyte sheet10 has a very textured side 20 and a less textured side 22,corresponding to the bottom of the casting and the top surface of thecasting respectively. The sintered electrolyte sheet 10 is freestanding-i.e., it can be handled without requiring additional support.

The invention will be further clarified by the following examples.

An anode ink was screen printed on the textured side of the sinteredelectrolyte sheet of this example, and a cathode was printed on theopposing side. During drying at 150° C. for 30 minutes the ink remainwell adhered to either side of the micro-textured electrolyte sheet andits adherence is better than its adherence to non-textured electrolytesheet of the same thickness.

FIG. 12 schematically illustrates a crossed single cell. The electrodeswere screen printed on a micro-textured zirconia—3 mole % yttriaelectrolyte sheet 10 illustrated in FIGS. 11A and 11B. Morespecifically, the crossed cell includes two crossed electrodes, eachelectrode being 2 cm long by 1 cm wide, resulting in effective crosssectional area of 1 cm². The electrodes were printed and fired on themicro-textured 3YSZ electrolyte sheet in successive operations. First ananode layer comprising a mixture of 3YSZ and nickel oxide was printedand fired at 1350° C. for 1 hour. Next a cathode comprising a mixture of3YSZ and lanthanum strontium manganate (LSM) was printed and fired at1200° C. for 1 hour. Next, a silver 10% palladium alloy mixed withdysprosium bismuthate cathode current collector was printed on thecathode side and a silver 10% palladium alloy mixed with 3YSZ anodecurrent collector was printed on the anode side. The current collectorswere co fired at 850° C. for 1 hour. The resultant single crossed cellwas tested in a simple “packet” configuration illustrated in FIG. 12.Forming gas (6% H₂—balance N₂) was provided to the interior chamberthrough a gas feed tube; air is supplied to the packet exterior. Whentested at 725° C. this cell provided power density of 0.39 W/cm². Incomparison, a similar cell was fabricated with identical electrodes,with a similar electrolyte sheet cast as above, but on a flat Teflon®surface. The resultant flat, untextured electrolyte sheet was uniformly20 micrometers thick. The cell with the flat, untextured electrolytesheet reached a maximum 0.32 W/cm² under similar test conditions.Therefore, electrolyte surface texturing improved electrolyte cellperformance by nearly 25%, when 6% H₂, balance N₂ forming gas mixturewas utilized.

Example 2

Applicants also discovered that it is desirable to modulate theelectrolyte thickness in a patterned fashion in order to improve its netmechanical properties. First consider the case of an electrolyte ofuniform thickness. If the space between electrode strips (i.e., viagalleries 55) has less printed material (to accommodate vias and/or viapads) the via gallery will be comparatively less “stiff” then theelectrode regions. On flexure of the device, the via gallery regionswill be subject to stress concentration because they are relatively moreflexible than the electrode regions and will have a relatively shortradius of curvature. In this case, because we wish to avoidconcentrating stress in the regions between the electrodes which containthe vias (i.e., through holes), a more uniform flexure is desiredthroughout the electrolyte sheet. Therefore, it will be advantageous toprovide more thickness in the via gallery regions, which will providemore uniform flexure of the electrolyte sheet.

FIG. 13A illustrates an electrolyte sheet with thickness variations forcontrol of mechanical flexure. FIG. 13B illustrates a cross-section ofthe portion of this electrolyte sheet. Thicker regions t₁ of theelectrolyte sheet will become the via galleries of the finished deviceand are separated by a distance of 10 mm. The thin regions t₂ will beprinted with the electrode layers. To achieve the desired deviceflexural properties, it is preferable that the thickness of theelectrolyte sheet corresponding to via galleries be 15 micrometers to 60micrometers and, preferably, 15 to 45 micrometers thick and even morepreferably 18 to 25 micrometers thick. For example, the thicker regionsmay be 60 micrometers thick while the thinner regions may be 20micrometers thick, which results in Δt of 40 micrometers.

Example 3

FIG. 14 illustrates another example of the electrolyte sheet 10 with atextured surface 20. Surface 20 includes a plurality of linearindentations or grooves. These features are 3 micrometers wide and 3micrometers deep and separated by 3 micrometers. The non-groovedportions of the electrolyte sheet 10 are 15 micrometers thick. Thegrooves reduce the average electrolyte thickness by 1.5 micrometers(10%) and increase the surface area by a factor of 2 (100%) in thetextured region.

A similar plurality of grooves with 1 micron separations, verticalwalls, 1 micrometer wide and 6 micrometers deep will have an aspectratio of 6:1 and thus reduce the average electrolyte sheet thickness by3.5 micrometers (20%) while increasing the surface area by a factor ofseven.

As another variation of this example, an array of grooves 3 micrometerswide and 5 micrometers deep with wall angles of 70 degrees reduces thethickness of the electrolyte sheet by 2.5 micrometer (about 17% of a 15micrometer nominal thickness) and increases the surface area by a factorof 2.34.

FIG. 15 shows a top view of similarly patterned electrolyte sheet asseen under a microscope. The pattern includes 3 micrometer wide channelswith depths of 3 micrometers, separations of 3 micrometers, and 70degree wall angles. These features reduce the thickness of theelectrolyte sheet by 1.5 micrometer (about 10% of a 15 micrometernominal thickness) and increase the surface area by a factor of 1.70.

It is preferable the surface area features increase the electrolytesurface area by a factor of 1.1 to more than 20. The preferable aspectratio of these surface features falls in the range of 0.1:1 to 10:1.

Example 4

A 75 micron thick Mylar® substrate carrier was coated with a thin, lessthan 1 micrometer layer of methyl cellulose as a release agent and driedat 65° C. for more than 1 hour. An acrylic layer was then cast with a12.5 micron or 25 micrometer clearance doctor blade over the methylcellulose layer and dried at 65° C. for more than ½ hour. Using a sharpblade (such as Exacto knife or a razor blade), a pattern similar to thatdepicted in FIG. 16A was made in the acrylic layer. The areas on thepattern that corresponds to the thick areas in the final electrolytesheet were the areas on the Mylar® substrate where the acrylic layerwere carefully pealed off. A second layer of methocel was applied to thepatterned acrylic and the substrate carrier and was also dried. Using a12.5-micrometer or a 25-micrometer gap tape casting doctor blade, aceramic slip was cast over the patterned acrylic on the substratecarrier to form a green sheet. After the green layer had dried, a secondacrylic layer (overcoat) was cast over the green sheet. After theovercoat dried, the green sheet with the acrylic overcoat was removedfrom the Mylar® substrate carrier. (The second acrylic layer is appliedto provide backing to the green sheet in order to enable it to beseparated (pulled off) from the methocel layer.) The green sheet withthin and thick portions was sintered in air at 1430° C. for 2 hoursresulting in a dense, flexible ceramic sheet 10 with thick and thinareas. The surface profile of this ceramic sheet was measured via SEMand surface profilimoter and we observed that the thin areas wereseveral to ten micrometers thinner than the thicker areas. Anelectrolyte sheet pattern similar to that of FIG. 16B may also beutilized.

Example 5

In a filtered air “clean” environment, Scotch® tape was placed on a flatglass substrate. The tape segments were about 25 micron thick. A 25micrometer thick Teflon® substrate carrier was placed upon the glasssubstrate and the tape segments and the Teflon® was smoothed to avoidwrinkles. Using a 50 micron gap tape casting “doctor” blade, a 40 inch×6inch sheet of ceramic/polymer (zirconia,—3 mole % yttria powder) wascast upon the Teflon® substrate carrier. The ceramic layer was dried for½ hour at room temperature under a plastic cover with about a 2-mm gapalong the 6-inch edges of the ceramic sheet but no gap along the longedges. Second, it was dried in an oven at 60 degrees C. for 1 hr. Theacrylic layer of the above described composition was cast on top of theceramic after drying using a 7 inch width, 25 micrometer gap blade.After the acrylic was dried at room temperature for ½ hour then 60° C.for 1 hour, the green ceramic with acrylic overcoat were removed fromthe Teflon® carrier. The green ceramic sheet with thin and thicker areaswas sintered at 1430° C. for 2 hours. After sintering the thin and thicklayers were easily observed by the amount of transparency in thesintered sheet. SEM observation gave the thickness of the thick areas as26 microns and the thickness of the thin areas as 15 microns.

Example 6

An electrolyte sheet, when utilized in a fuel cell, will typically beoperated with much higher flow of gas (air or oxygen), compared to fuelflow. This is done to supply sufficient oxygen from the air to the fuelcell assembly. The airflow may create a greater pressure and stress inmiddle region of the electrolyte sheet. More specifically, anelectrolyte sheet will experience a predominately compressive force onthe high pressure side (for example, air side) and a predominatelytensile force on the other (for example, fuel side). Applicant's foundthat if the electrolyte sheet has one textured and one relatively smoothsurface, it is preferable for the electrolyte sheet to be oriented in amanner such that the textured surface experiences predominatelycompressive forces. Thus, it is preferable that the patterned side ofthe electrolyte sheet faces the air (or oxygen). The surface features(such as small surface defects and/or protruding features of thetextured surface, when subjected to predominately tensile forces mayresult in stress concentration and tearing around these features.Therefore, it is preferable that the relatively smooth side of theelectrolyte sheet, rather than the more textured side, experiences thepredominately tensile forces. Thus, it is preferable that the relativelysmooth surface of the electrolyte sheet faces the fuel side of the fuelcell device. In addition, because cathode act as relatively poorcatalysts, as compared to anodes, the cathode facing side of theelectrolyte sheet will benefit more from the increased surface area thenthe anode facing side of the electrolyte sheet. Furthermore, the cathodefacing side of the electrolyte sheet is also the air (oxygen) facingside. A relatively smooth surface or side or the electrolyte sheet isthe side or surface of the electrolyte sheet with finer surface featuresthan the more textured surface or side. For example the smooth side mayhave 1 micron tall features while the more textured side may have 10micron features. Alternatively, the smooth side of the electrolyte maynot be textured.

Therefore, texturing of the electrolyte sheet can result in a thinnerelectrolyte sheet and thus reduced ohmic resistance and higherefficiency, while orienting the electrolyte sheet with a more texturedsurface facing the air (oxygen) and the less textured surface facing thefuel, results in better mechanical durability.

If the electrolyte sheet is held in a frame 111, it may experiencebuckling, as shown schematically in FIG. 17A. An embodiment of thetextured electrolyte sheet that is especially suited to operate in suchenvironment is illustrated schematically on FIG. 17B. As otherelectrolyte sheets examples disclosed above, this electrolyte sheetincludes thicker and thinner areas. However, in the electrolyte sheet ofthis embodiment, the thinner areas become progressively thinner closerto the edges. That is, because some regions of the electrolyte sheet(such as the center, for example) experience higher stresses whenpressurized, it is advantageous that these regions of the electrolytesheet have larger average thickness than the regions experiencing lessstress.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. An electrolyte sheet comprising a body of varied thickness, saidelectrolyte sheet having a textured surface with multiple protrudingfeatures, said protruding features forming an undercut angle withrespect to the normal of said electrolyte sheet, said undercut anglebeing more than 0 degrees and less than 15 degrees.
 2. The electrolytesheet of claim 1, wherein said undercut angle is 1 to 10 degrees.
 3. Theelectrolyte sheet of claim 1, wherein the thickest part of saidelectrolyte sheet is at least 0.5 microns greater than the thinnest partof said electrolyte sheet.
 4. The electrolyte sheet of claim 1, whereinthe electrolyte sheet is a ceramic sheet formed of a polycrystallineceramic selected from a group consisting of partially stabilizedzirconia or stabilized zirconia, and being doped with a dopant selectedfrom the group consisting of the oxides of Y, Ce, Ca, Mg, Sc, Nd, Sm,Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, In, Ti, Sn, Nb, Ta, Mo, W andmixtures thereof.
 5. The fuel cell device of claim 1, wherein thethickest part of said electrolyte sheet is at least 2 micrometersgreater than the thinnest part of said electrolyte sheet.
 6. A solidoxide electrode/electrolyte assembly comprising: the electrolyte sheetaccording to claim 1, said electrolyte sheet having an averageelectrolyte sheet thickness between 0.5 micrometers and 45 micrometers,;at least one cathode disposed on a one surface of said electrolytesheet; and at least one anode disposed opposite the cathode on anothersurface of said electrolyte sheet.
 7. A solid oxideelectrode/electrolyte assembly according to claim 6, comprising aplurality of cathodes situated on one side of said electrolyte sheet anda plurality of anodes situated on an opposite side of said electrolytesheet.